During a significant frontal impact to a motor vehicle, the passenger(s) is accelerated forwards in response to the impact force. The risk of injury may be greatly reduced by a highly placed level of support (e.g., such as an upraised knee bolster) at the front edge of the seat cushion. In particular, this upraised portion also significantly improves the restraining action of the seat belt (when properly adjusted) and prevents the passenger from slipping through or "submarining" beneath the seat belt. In normal driving conditions, however, the knee bolster should remain in a lowered position for reasons of seating comfort.
From French patent document FR-PS 1 012 718, it is known how to tip a vehicle seat backwards, about a transverse axis of the motor vehicle in the event of a frontal impact. This backwards tipping movement of the seat is executed by a connecting rod having a first forward end attached to the front fender and a second rearward end extending into the passenger compartment and connected to the seat. In the event of a frontal impact, the connecting rod is moved into the passenger compartment and detaches the seat from its front support, thus tipping the seat such that it rotates generally backwards about a transverse axis passing through the rear seat support. However, for other safety considerations, a connecting rod which penetrates into the passenger compartment during the frontal collision is unfavorable as it presents a safety hazard to the occupant. Further, the actuation is unreliable because the connecting rod is prone to bending and jamming during its rearward movement. A further disadvantage is that the seat back also tilts backward, thus leaving the passenger's back unsupported during impact.
In German patent document DE-PS 36 31 881, there is disclosed a front edge section of a vehicle seat which is stiffened during a significant frontal impact to a motor vehicle such that the weight of an occupant cannot push the front edge section down. The stiffening action is achieved by a stretching gear, which is activated by either a pyrotechnical linear drive or a pyrotechnical rotational drive. An acceleration sensor, located at the front of the vehicle, senses the frontal impact and actuates the selected drive device. In view of all the specified components including the acceleration sensor, the pyrotechnical drive, the necessary actuating cables, and perhaps other redundant elements, such a design is overly elaborate, complex, and costly.
German patent document DE-OS 33 37 232 discloses another arrangement wherein the front edge section of a vehicle seat is raised by a lifting mechanism upon a frontal impact to a vehicle. The lifting mechanism is activated through utilization of the impact energy of the vehicle and by the relative movement of parts of the body across motion transmitting elements. This lifting apparatus utilizes the relative movement between an assembly arranged in the front of the vehicle which is caused to move rearward towards the passenger compartment by the impact deformation experienced in a frontal collision. The assembly is connected to the lifting mechanism via a cable and pulley system, so that as the assembly is moved rearward, the front edge section of the seat is raised.
There are several shortcomings with this device. For example, it is difficult to connect the lifting mechanism to the assembly, which is initially located in the front of the vehicle, because of the relatively long distances involved. Further, it is costly to build a device that is active during the frontal impact yet inactive when the seat is merely adjusted to accommodate the passenger's sitting posture and comfort requirements.
Another prior art safety mechanism for a vehicle seat is disclosed in French Patent FR-A 2 261 158, whereby a lifting mechanism is used to raise the front edge section of a vehicle seat in order to improve the supporting action of the seat cushion by enlarging the effective wedge angle. The lifting mechanism is activated by utilizing the impact energy of the vehicle through the relative motion of crumpling vehicle parts with respect to a stationary pivot joint. The lifting mechanism includes a pair of longitudinally disposed guiderails to which a rear portion of the seat is attached via a set of rollers such that the seat can slide in a lengthwise or longitudinal direction, along the guiderails. A support is provided with one end hinged to the stationary pivot joint attached to the vehicle floor and the other end pivotally attached to a seat support connection disposed on a lower middle portion of the seat cushion. In the normal operating position of the vehicle seat, the support is in a generally lowered position, since the stationary pivot joint is disposed in front of the support connection. The support is held in this position by a spring. During a frontal collision, an inertia force associated with the seat and the passenger sitting thereon is sufficient to overcome the spring force. As a result, the seat moves forward and the support is pivoted generally upward thereby raising the seat's front edge.
A shortcoming of this arrangement is that in operation, the seat back becomes tilted too far backward thereby losing its supporting action. Furthermore, the lengthwise guiderails are part of the lifting mechanism and cannot be used for seat adjustment purposes when an adjustment in lengthwise direction is desired solely for comfort reasons. Additional guiderails are required to accommodate this purpose thereby increasing the complexity and cost of the device.
Thus, there is a definite and as yet unfulfilled need for an inexpensive, simple, and effective device for elevating a front edge section of a vehicle seat during a significant frontal impact to a motor vehicle in a safe manner and which overcomes the problems of the prior art.